The universe is made up of galaxies and stars.
The universe exploded 15 billion years ago. This explosion sent the planets in different directions. Billions of years later, when astronomers looked at the universe, they saw that the galaxies and planets were moving away from each other. This means that the universe is expanding.
If that happens, then the old stars could disappear and there may not be any new stars. Also, everything in space could turn dark and become much colder. The universe could suddenly end with an explosion.
Instead, if all the galaxies and their stars moved closer to each other, everything could smash together and the universe would end. This would be the opposite of what happened in the Big Bang, a gigantic explosion that started the world.
[Source: 100 Things You Should Know About Space]